Jpn. J. Appl. Phys., Vol. 39 (2000), pp. L647-650 teaches a way of improving an output and a lifetime of a nitride semiconductor laser device, wherein a GaN underlayer is grown to cover a mask pattern of uniform SiO2 stripes formed over an entire region of one main surface of a GaN substrate and windows unprovided with the SiO2 masks, and the nitride semiconductor laser device is formed on the GaN underlayer.
According to the article, threading dislocations in the GaN underlayer decrease in a region above the SiO2 mask, and then the output and the lifetime of the laser device can be improved by utilizing the underlayer region having such a small threading dislocation density.
However, it is still desired to improve the lasing lifetime even in the nitride semiconductor laser device disclosed in the above-described article Jpn. J. Appl. Phys., Vol. 39 (2000), pp. L647-650.
Incidentally, there are different two ways to provide a pair of electrodes to a semiconductor laser chip. In the first way, two electrodes (a p-side electrode and an n-side electrode) opposite to each other are provided on a front side of a semiconductor laser device structure formed on a substrate and on a back side of the substrate, respectively. Hereinafter, such an arrangement of electrodes is called “counter electrodes arrangement”. In the second way, a p-side electrode (or an n-side electrode) is provided on a front side of a semiconductor laser device structure formed on a substrate, and a portion of an n type layer (or a p type layer) is exposed by reactive ion etching on the front side of the device structure to provide an n-side electrode (or a p-side electrode) on the exposed region. That is, both the p-side electrode and the n-side electrode are provided on the same side of the substrate. Hereinafter, such an arrangement of electrodes is called “one-side electrodes arrangement”.
In comparison between the counter electrodes arrangement and the one-side electrodes arrangement, while it is necessary to secure both the regions for forming the p-side and n-side electrodes on the front side of the substrate in the one-side electrodes arrangement, the back side of the substrate can be used as the electrode formation region in the counter electrodes arrangement. That is, in the laser chip having the counter electrodes arrangement, the substrate area can be used more efficiently, allowing reduction of the chip size. Although formation of a laser chip having the one-side electrodes arrangement requires reactive ion etching of a portion of the semiconductor laser device structure, such a complicated process is unnecessary to form a laser chip having the counter electrodes arrangement.
In the nitride-based semiconductor laser device having the counter electrodes arrangement, however, its threshold voltage is liable to increase considerably compared to the case of the laser device having the one-side electrodes arrangement. Thus, it is common to employ the one-side electrodes arrangement in a semiconductor laser device.
Under these circumstances, it is also desired to decrease the threshold voltage of the semiconductor laser device having the counter electrodes arrangement in order to promote utilization of the semiconductor laser device with the counter electrodes arrangement which has the advantages of reduced chip size and simplified manufacturing process.
In view of the foregoing, a primary object of the present invention is to further improve nitride semiconductor light emitting devices in their light-emitting lifetimes, luminous intensities, yields and others. Another object of the present invention is to further improve nitride semiconductor laser devices having the counter electrodes arrangement in their threshold voltages and others.